Due to their exceptional physiochemical properties, carbon nanotubes (CNT) raise great industrial and research interests in fields as diverse as biomedicine, microelectronics, or composites. Surface functionalization by acid treatment has been described as an easy way to reduce the CNT hydrophobicity and biopersistance. Its impact on in vitro cellular response remained however unclear with numerous contradictions between studies. This work aimed to provide a comprehensive analysis of the acid functionalization effects on cellular activity. The impact of the CNT purification from their catalytic impurities and the presence of surface acid groups have been discriminated. A thermal desorption treatment to remove surface acid groups has been conducted after acid functionalization by HNO₃, H₂SO₄. After a complete physicochemical characterization, the biological toxicity was evaluated on RAW 264.7 murine macrophages. Results showed that acid functionalization leads to a decrease of the pro-inflammatory response, oxidative stress and to a lesser extent cytotoxicity due to the CNT purification, and an increase of the pro-inflammatory response due to the surface acid groups. This study, published in Journal of Nanoparticle Research ¹, underlined the necessity to weight the benefit-risk balance of such an acid purification based on the CNT primary state of purification for a safer-design approch. 1. Figarol, A. et al. Biological response to purification and acid functionalization of carbon nanotubes. J. Nanoparticle Res. 16, 1–12 (2014).